Gear pump bearing

ABSTRACT

Invention provides gear pump bearing block and method of manufacturing gear pump bearing block. Bearing block includes a bush formed of antifriction alloy, bush having cylindrical portion providing bore adapted to receive bearing shaft of gear of pump, and further having flange portion extending radially outwardly at end of cylindrical portion to provide thrust face adapted to slidingly engage with side surface of gear. Bearing block also has backing layer covering radially outer surface of cylindrical portion and rear face of flange portion, backing layer being formed of less dense alloy compared to antifriction alloy. Furthermore, there is an annular gallery embedded in flange portion such that gallery is spaced from surface of bore and from thrust face, gallery surrounding bore, with inlet to and outlet from annular gallery such that, in use, fluid flows from inlet, through annular gallery to provide cooling of thrust face, and then to outlet.

FIELD OF THE INVENTION

The present invention relates to gear pump bearings. In particular, butnot exclusively, the invention relates to improvements in gear pumpbearings for use in aero-engine fuel supply systems.

BACKGROUND

In a gas turbine engine fuel delivery system, pump assemblies, as shownfor example in US 2005/0232784, are typically used for pumping the fuel.Where such assemblies include gear pump, gear elements are commonlysupported by bearing blocks which are adapted to receive respectivebearing shafts of the gears through a bore of each bearing block. Thesebearing blocks also typically abut axially-directed faces of respectivegears of the pumps. The bearing blocks may be for solid bearings, orpressure loaded bearings. A solid bearing typically transfers load fromjournals to the pump housing, and additionally can transfer axial loadto the housing. Pressure loaded bearings also transfer load fromjournals to housing, and in addition can provide an axial force and amoment against the axially-directed face of the gear which the bearingblock abuts.

It is known to use bimetallic (alloy) bearing blocks, as shown, forexample, in U.S. Pat. No. 4,523,365. Such a bimetallic bearing blockgenerally comprises an inner bush covered with an outer backing layer.The inner bush is formed of an alloy which provides a tribologicallycompatible surface for the gear side face and journal to run against.However it may be undesirable for the whole bearing block to be formedof such an alloy. Two reasons for this are that firstly the weight of ablock formed solely of such an alloy may be larger than desired, andsecondly the difference between the coefficient of thermal expansion(CTE) and that of the light alloy normally used for the pump housingbody may be large. Therefore, in a bimetallic bearing block the innerbush is coated with a light alloy backing layer, which reduces theoverall weight of the block, and mitigates the CTE difference with thepump housing body.

In order to provide a tribologically compatible surface for the gearside face and journal to run against, an antifriction alloy is typicallyused to form the inner bush. The antifriction alloy may be, for example,a lead bronze alloy. In particular, the antifriction alloy may be a highlead bronze alloy, with a lead concentration of typically 20-30 wt %,although the alloy composition is not limited to this range, and may beselected according to the desired material properties required for thebearing block and the particular application for which it is being used.

However, testing of bearing blocks which use such an antifriction alloyhas shown that such blocks can be prone to suffer permanent radialdeformation along the bearing bore when operated above a thresholdpressure/temperature combination. If this permanent radial deformationis significant, it can reduce the clearance between the gear journal andthe bearing bore. As a result of this reduced clearance, overheating canoccur, which may result in mechanical damage of the gear and/or bearing.This problem is more evident in bearing blocks having a higherconcentration of lead (e.g. 30 wt % lead bronze bearings), although thesame problem may also occur to some extent in bearings having adifferent alloy composition (e.g. in 20 wt % lead bronze bearings) ormade from a different antifriction alloy.

One possible solution to the problem of preventing radial deformation isto use materials having increased strength. Lead-bronze alloys whichhave a lower lead content, e.g. 20 wt % lead bronze, have a higher yieldstress and tend to suffer less permanent deformation in use than leadbronze alloys with a higher lead content, e.g. 30 wt % lead bronze.However, bearings made of 20 wt % lead bronze have also been shown tohave poorer performance as thrust bearings than those manufactured from30 wt % lead-bronze, and suffer a problem of poor load carrying capacityat the thrust face of the bearing block. One reason for this poorerperformance may be due to the relationship between thermal conductivityof the material and the wear properties of the material.

FIG. 1 shows a typical Stribeck curve, which describes how coefficientof friction varies for different lubrication regimes. It can be seenthat typically, higher coefficients of friction occur in a mixed modelubrication regime as compared to in a full-film lubrication regime. Thethrust faces in a gear pump bearing arrangement typically operate atleast partly in a mixed-film lubrication regime. This is evident fromthe wear and scoring that can be visualised at the thrust surfaces afterrunning. In contrast, the journal element of the bearing typicallyoperates in a full-film lubrication regime. Generally, the loads on thethrust face of a bearing block are lower than those acting on thejournal element of the bearing, but the sliding velocities are higher.Accordingly, local heating of the thrust face and gear side face is morelikely than in the journal element of the bearing. The situation may beparticularly acute at the region of the thrust face under the gear rootcircle diameter where fluid cooling is limited by the restricted fluidflow that occurs across this section of the thrust face. The primarymode for heat transfer away from the thrust face at this point may beconduction through the inner bush and backing layer of the bearingblock. Therefore, depending on the thermal conductivity of the alloys ofthese components, there may be a problem that heat cannot transfer awayfrom the thrust face sufficiently quickly. This may then lead to reducedwear characteristics, and correspondingly poorer performance of thethrust face in a thrust bearing capacity.

SUMMARY

The present invention aims to address the above problems.

Thus, in a first aspect, the present invention provides a gear pumpbearing block having:

-   -   a bush formed of antifriction alloy, the bush having a        cylindrical portion providing a bore adapted to receive a        bearing shaft of a gear of the pump, and further having a flange        portion extending radially outwardly at an end of the        cylindrical portion to provide a thrust face adapted to        slidingly engage with a side surface of the gear;    -   a backing layer covering a radially outer surface of the        cylindrical portion and a rear face of the flange portion, the        backing layer being formed of relatively less dense alloy        compared to the antifriction alloy;    -   an annular gallery embedded in the flange portion such that the        gallery is spaced from the surface of the bore and from the        thrust face, the gallery surrounding the bore; and    -   an inlet to and an outlet from the annular gallery such that, in        use, fluid flows from the inlet, through the annular gallery to        provide cooling of the thrust face, and then to the outlet.

Typically the cooling fluid is also the pumped fluid. However, in somecases the cooling fluid may be a fluid that is different from the pumpedfluid.

In a second aspect, the present invention provides a method ofmanufacturing a gear pump bearing block, the method including steps of:

-   -   providing a bush formed of antifriction alloy, the bush having a        cylindrical portion providing a bore adapted to receive a        bearing shaft of a gear of the pump, and further having a flange        portion extending radially outwardly at an end of the        cylindrical portion to provide a thrust face adapted to        slidingly engage with a side surface of the gear;    -   forming an annular groove in the rear face of the flange portion        and surrounding the bore, the groove being spaced from the        surface of the bore and from the thrust face;    -   partially filling the annular groove with an annular insert        having a depth less than the depth of the groove, thereby        forming an enclosed volume constituting an annular gallery        between a bottom face of the insert and the floor of the groove;    -   forming an inlet to and an outlet from the annular gallery such        that, in use, fluid flows from the inlet, through the annular        gallery to provide cooling of the thrust face, and then to the        outlet; and    -   covering a radially outer surface of the cylindrical portion and        the rear face of the flange portion of the bush with a backing        layer, the backing layer being formed of relatively less dense        alloy compared to the antifriction alloy.

It will be noted that the above method steps are not necessarily limitedto being performed in the order as stated above. For example, the stepof forming the inlet to and the outlet from the annular gallery may beperformed before or after the step of covering the radially outersurface of the cylindrical portion and the rear face of the flangeportion of the bush with the backing layer.

The present invention thereby provides a gear pump bearing block and amethod of manufacturing a gear pump bearing block having means forcooling the thrust face of the bearing block in the form of an annulargallery embedded in the flange portion of the bearing block bush.Accordingly, heat transfer away from the thrust face of the bearingblock may be improved. By increasing the heat transfer away from thethrust face, the load carrying capacity of the thrust face may becorrespondingly improved. This can allow materials with increasedstrength but less inherent load capability to be used. For example, alower lead content bronze may be used as the antifriction alloy of thebush rather than a high lead content bronze. Use of such materials withincreased strength can thereby help to prevent significant radialdeformation of the bore of the bearing block.

In a third aspect the invention provides a gear pump having one or moregears with bearing shafts supported by respective gear pump bearingblocks of the first aspect.

In a fourth aspect the invention provides a fuel supply system of a gasturbine engine having the gear pump according to the third aspect forpumping fuel. For example, the fuel supply system may include a dualstage pump formed of a low pressure pump and a high pressure pump. Thelow pressure pump may be a centrifugal pump. The high pressure pump maybe the gear pump according to the third aspect.

In a fifth aspect, the invention provides a gas turbine engine havingthe fuel supply system of the fourth aspect.

Optional features of the invention will now be set out. These areapplicable singly or in any combination with any aspect of theinvention.

The gear pump bearing block may further have a recess in the surface ofthe bore which forms, in use, a hydrostatic pad for the supply of fluidto the interface between the bore surface and the bearing shaft, theoutlet from the annular gallery being formed in the recess. Highpressure fluid, e.g. diverted from the discharge of the gear pump, maythus be fed to the hydrostatic pad via the annular gallery formed in thebearing block. The hydrostatic pressure formed in this hydrostatic padcan assist in achieving full-film lubrication at the journal element ofthe bearing block (i.e. inside the bearing bore, between the gearbearing shaft and the bore surface). The flow exhausting from thehydrostatic pad may also provide forced cooling of the bearing blockwithin the bearing bore. By using the flow in the annular gallery tolubricate the journal element and provide such forced cooling aftercooling the thrust face, the amount of flow diverted from the dischargeof the gear pump can be reduced. In the context of an aeroengine fuelpump, this may help to ensure that the gear pump has sufficient flowcapacity at the low speeds associated with the windmill relightcondition. Put another way, reducing the amount of diverted flow meansthat the size and weight of the pump may not need to be increased.

The inlet to the annular gallery may be formed at the thrust face.However, the inlet is not limited to being formed at a gear-contactingportion of the thrust face. For example, where portions of the thrustface are set back so as to provide side ports which allow fluid into andout of meshing gears, the inlet to the annular gallery may convenientlybe formed at such a side port. This may improve fluid flow into theinlet compared to a case where the inlet is formed at a gear-contactingsurface of the bearing block.

The gear pump bearing block may further have a bearing bridge insert(see U.S. Pat. No. 7,607,906) at the thrust face partitioning highpressure and low pressure sides of the gear and providing locallyincreased cavitation erosion resistance. The bearing bridge insert maybe formed of, for example aluminium bronze, or any other suitablematerial. The inlet to the annular gallery may be formed on the highpressure side of the bearing bridge insert. The bearing bridge insertmay intersect the annular gallery. In this case, there may be a passageformed in the insert to allow fluid communication of two legs of theannular gallery through the insert.

This can help to ensure that both legs of the annular gallery are fedwith an approximately equal delivery pressure and are not left stagnant.

The annular gallery may be located at the same radial position as a gearteeth root circle of the gear which, in use, slidingly engages with thethrust face. This can help to ensure cooling of this typicallyproblematic region of the thrust face.

The annular gallery may be located 2% or more and/or 20% or less of theaxial length of the bush from the thrust face, as measured in the axialdirection of the bore. The annular gallery may be spaced a distanceradially outwards of the surface of the bore, which distance is at least15% of the radial distance between that surface and the outer radius ofthe flange portion. The cross-sectional area of the annular gallery mayvary around its circumference. This may be achieved by varying the depthof the annular groove formed in the rear face of the flange portion ofthe bush, and/or by varying the width of the groove around thecircumference of the bearing block. By varying the cross-section of theannular gallery in this way, the mechanical strength of the bearingblock can be altered, along with the flow velocity and heat transfercoefficients around the annular gallery. The amount of heat transfer isrelated to the flow number in the cooling gallery.

The antifriction alloy from which the bush is formed may be a leadbronze alloy although other alloys can be used. Where a lead bronzealloy is used, the lead content of the alloy may be selected accordingto the desired material properties of the antifriction bush, but forexample the alloy may have a lead content of between 20 wt % and 30 wt %lead.

The bush may have an antifriction coating of a different material to theantifriction alloy e.g. on the surface of the bore and/or the thrustface. For example, the antifriction alloy may be lead-indium plated orcoated with a dry film lubricant. This can increase the tribologicalcompatibility of the bush and the bearing shaft.

The step of covering the radially outer surface of the cylindricalportion and the rear face of the flange portion of the bush with thebacking layer may be performed by thermally-spraying the relatively lessdense alloy onto the antifriction bush. Conveniently, the thermalspraying technique may be flame-spraying, as described in U.S. Pat. No.4,523,365. The relatively less-dense alloy may be, for example,aluminium alloy, although this is not particularly limited.

The annular gallery may be formed by partially filling an annular grooveformed in the rear face of the flange portion with an annular inserthaving a depth less than the depth of the groove, thereby forming anenclosed volume constituting the annular gallery between a bottom faceof the insert and the floor of the groove. However, the method offorming the annular gallery is not particularly limited to such aprocess step, and any suitable process capable of forming an annulargallery embedded in the flange portion such that the gallery is spacedfrom the surface of the bore and from the thrust face may be used. Wherethe annular gallery is formed by partially filling an annular groove inthe rear face of the flange portion with an annular insert, the insertand bush may be machined together to form a continuous,smoothly-profiled rear face. This can assist in providing a smoothsurface onto which the backing material can then be applied to form thebacking layer. The material from which the insert is formed is notparticularly limited, however the insert may be formed of theantifriction alloy.

Once the annular insert is located in the flange portion of the bush,there may be a step of hot isostatic pressing (‘HIPing’) the block. Thismay help to prevent leakage paths at the boundaries between differentcomponents, in particular around the periphery of the annular insert.The step of HIPing may be performed before or after the step of coveringthe radially outer surface of the cylindrical portion and the rear faceof the flange portion of the bush with the backing layer.

The block may further have an annular formation of one or morestiffening members, the formation surrounding the bore, the, or each,stiffening member being embedded in the flange portion, and the, oreach, stiffening member being formed of a material having a higherelastic modulus than the antifriction alloy. The one or more stiffeningmembers can reduce the stresses in the bush adjacent the bore.Additionally, it is possible to provide this advantage without having tosacrifice the performance of the antifriction alloy. Thus, for example,a high lead content bronze alloy having lower yield strength butimproved wear performance may be used as the antifriction alloy ratherthan a lower lead content bronze alloy having a higher yield strengthbut reduced wear performance.

In order to manufacture a block having such a stiffening member(s), theabove method may further include: forming one or more recesses in anannular arrangement in the rear face of the flange portion andsurrounding the bore; and locating one or more stiffening members inrespective of the recesses, the, or each, stiffening member being formedof a material having a higher elastic modulus than the antifrictionalloy. The steps of forming the one or more receiving recesses, andlocating the one or more stiffening members in respective recesses maybe performed before or after the step of covering the radially outersurface of the cylindrical portion and the rear face of the flangeportion of the bush with the backing layer.

The stiffening member(s) may be spaced from the annular gallery. Thus inthe above method, the annular groove used to form the gallery may beradially spaced from the recess(es) in which the stiffening member(s) islocated. However, another option is to locate an annular stiffeningmember in the annular groove used to form the gallery, i.e. to use thegroove for the gallery as the recess for the stiffening member. Thus theannular gallery may be formed by partially filling the annular grooveformed in the rear face of the flange portion with an annular stiffeningmember having a depth less than the depth of the groove, thereby formingan enclosed volume constituting the annular gallery between a bottomface of the stiffening member and the floor of the groove.

The, or each stiffening member may be spaced from the surface of thebore and/or from the thrust face. In this way, exposure of thestiffening members at the surface of the bore, and/or at the surface ofthe thrust face can be avoided. This has an advantage that surfaces ofthe bearing block which contact the gear bearing shaft and gear sidesurface (at the bore and the thrust face of the bearing blockrespectively) may be continuously formed of appropriate antifrictionalloy. Also, surface exposure of the stiffening member(s) can lead tothe formation of fluid leakage paths through the bush.

The, or each, stiffening member may be spaced 2% or more and/or 20% orless of the axial length of the bush from the thrust face, as measuredin the axial direction of the bore. The stiffening effect may be tunedby varying the distance of the stiffening member(s) from the thrust facearound the circumference of the bore. A spacing of more than 20% maylead to a reduction in the effectiveness of the stiffening member(s) toprevent deformation of the bearing block. A spacing of less than 2% fromthe thrust face may lead to problems of uneven wear performance of thatface.

The, or each stiffening member may be spaced a distance radiallyoutwards of the surface of the bore, which distance is at least 15% ofthe radial distance between that surface and the outer radius of theflange portion. In this way a controlled amount of elastic and/orplastic deformation of the bush in the region close to the bore can beallowed to accommodate bending and misalignment. Allowing the boresurface to become conformal with the bearing shaft in thecircumferential direction through such deformation can increase the loadcarrying capability of a lubricating fluid film between the bore surfaceand the shaft.

The gear pump bearing block may have a single annular stiffening member.For example, the stiffening member may be a ring which is positionedsymmetrically around the axis of the bore. Such a ring may be embeddedinto a ring-shaped recess formed in the rear face of the flange portionby, for example, trepanning, or any other suitable manufacturing method.

Alternatively, the gear pump bearing block may have a plurality ofcircumferentially spaced stiffening members. Conveniently thesestiffening member may be rods, although the form of the stiffeningmembers is not particularly limited. The axis of each rod may besubstantially parallel to the axis of the bore. The stiffening membersmay be regularly or irregularly spaced. There may be gaps in the annulararray e.g. corresponding to the positions of side ports formed in thethrust face. By varying the distribution of the stiffening members,their diameter and their depth around the circumference of the bearingblock, the radial stiffness of the bearing block can be varied to giveimproved performance.

The entirety of the stiffening member(s) may be embedded within theflange portion. Alternatively, the stiffening member(s) may extend fromthe flange portion into the backing layer. In this way, the stiffeningmember(s) may transfer some of the load from the flange portion into thegenerally stronger material of the backing layer, e.g. by a combinationof shear and bending.

The material from which the stiffening member(s) is formed is notparticularly limited, provided that the elastic modulus is higher thanthat of the antifriction alloy material from which the bush is formed.The material may also be a material which has a higher yield strengththan the antifriction alloy. One example of a material which may beparticularly suitable for forming stiffening members is steel. In thecase where there are a plurality of stiffening members, the stiffeningmembers may not all be formed of the same material. This can allow thestiffening effect provided by the stiffening members to be moreprecisely tuned by providing stiffening members which e.g. vary inelastic modulus around the circumference of the bore. The, or each,stiffening member may be formed of a material which has the same orsimilar coefficient of thermal expansion as the antifriction alloy. Forexample, the, or each, stiffening member may be formed of a materialwhich, within ±15%, or within ±10%, or within ±5% has the same orsimilar coefficient of thermal expansion as the antifriction alloy. Thiscan reduce the generation of significant differential thermal stresseswithin the bearing block.

The stiffening member(s) may be retained in their respective recessesformed in the rear face of the flange portion by application of thebacking layer over the stiffening members. The stiffening member(s) andbush may be machined together to ensure a continuous, smoothly-profiledrear face of the flange portion onto which the backing material can beapplied. Alternatively or additionally, the stiffening member(s) may beretained by a variety of other methods including, but not limited to,screwing, interference fitting, adhesive or Lee Plugs™.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows a typical Stribeck curve;

FIG. 2 shows a longitudinal cross-section through a ducted fan gasturbine engine;

FIG. 3 shows a cut away perspective view of the interior of a pumpassembly of the engine;

FIG. 4 shows an exploded perspective view of some components of the pumpassembly;

FIG. 5 shows a perspective view of a solid bearing block set;

FIG. 6 shows a perspective view of a pressure loaded bearing block set;

FIGS. 7A and 7B show a longitudinal cross section and a transverse crosssection on plane A-A through a solid bearing block having an annulargallery embedded in a flange portion of a bush part of the block;

FIG. 8 shows a plan view of a portion of the thrust face of the bearingblock of FIG. 8 at a bearing bridge insert;

FIG. 9 shows a graph of modelled stress normalised by yield stressagainst normalised radial distance (r/r₀) for a bearing block having asingle annular stiffening ring; and

FIGS. 10A and 10B show a longitudinal cross section and a transversecross section on plane B-B through a solid bearing block having aplurality of circumferentially spaced stiffening members and an annulargallery embedded in a flange portion of a bush part of the block.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES

Although a gear pump bearing block, gear pump and fuel delivery systemof the present invention may be used in various applications, asignificant intended use is in an aircraft fuel supply system, and theinvention will be described hereafter in relation to such a system.

With reference to FIG. 2, a ducted fan gas turbine engine incorporatingthe invention is generally indicated at 10 and has a principal androtational axis X-X. The engine comprises, in axial flow series, an airintake 11, a propulsive fan 12, an intermediate pressure compressor 13,a high-pressure compressor 14, combustion equipment 15, a high-pressureturbine 16, an intermediate pressure turbine 17, a low-pressure turbine18 and a core engine exhaust nozzle 19. A nacelle 21 generally surroundsthe engine 10 and defines the intake 11, a bypass duct 22 and a bypassexhaust nozzle 23.

During operation, air entering the intake 11 is accelerated by the fan12 to produce two air flows: a first air flow A into theintermediate-pressure compressor 13 and a second air flow B which passesthrough the bypass duct 22 to provide propulsive thrust. Theintermediate-pressure compressor 13 compresses the air flow A directedinto it before delivering that air to the high-pressure compressor 14where further compression takes place.

The compressed air exhausted from the high-pressure compressor 14 isdirected into the combustion equipment 15 where it is mixed with fueland the mixture combusted. The resultant hot combustion products thenexpand through, and thereby drive the high, intermediate andlow-pressure turbines 16, 17, 18 before being exhausted through thenozzle 19 to provide additional propulsive thrust. The high,intermediate and low-pressure turbines respectively drive the high andintermediate-pressure compressors 14, 13 and the fan 12 by suitableinterconnecting shafts.

FIG. 3 shows a cut away perspective view of the interior of a dual stagepump assembly of a fuel supply system of the engine 10, and FIG. 4 showsan exploded perspective view of displacement gear components of theassembly of FIG. 3. The pump assembly has in sequence: an outer casingcomprising a mounting flange 25; a housing 27 for a smaller primarydisplacement gear pump 30; a housing 29 for a larger secondarydisplacement gear pump 32; a centrifugal stage back plate 34 which actsas an end cover for the housing 29 and additionally as a back plate fora centrifugal pump 36; and finally at the end of the casing remote fromthe mounting flange 25 a low pressure stage housing 38 for thecentrifugal pump 36, this housing including the centrifugal pump inlet.The centrifugal pump forms the low pressure stage of the dual stage pumpassembly, and the two gear pumps form the high pressure stage of theassembly. Typically, the small primary displacement pump 30 ispressurised at all flight conditions, while the large secondarydisplacement pump 32 is pressurised for high power (above cruise)conditions, and for low speed starting.

A drive shaft 40 which accepts power from an engine accessory gearbox(not shown) has male spline couplings at each end. The drive shaft 40accommodates for misalignment and connects directly into a driver gear44 of the secondary gear pump 32, and continues via a linking driveshaft 42 to the impeller and inducer of the centrifugal pump 36. Asecondary drive shaft 41 transfers power from the secondary pump to theprimary pump 30 and also accommodates for misalignment. Moreparticularly, one splined end of the secondary drive shaft is engagedinternally with the driven gear 45 of the larger, secondary displacementpump, whilst its opposite splined end is engaged internally with thedriver gear 46 of the smaller, primary gear pump, which drives thedriven gear 47 of the primary gear pump.

Each displacement pump gear 44-47 has a respective solid bearing block48 and a respective pressure-loaded bearing block 50 which are adaptedto receive a bearing shaft or journal of that gear. The bearing blocksare shown in greater detail in FIG. 5 and FIG. 6.

FIG. 5 shows a perspective view of two of the solid bearing blocks 48which form a solid bearing set. The solid bearings transfer load fromtheir gear journals to the outer casing, and also transfer axial thrustload to the casing. Each bearing block of the set has a bush 52 formedof antifriction alloy (e.g. lead bronze), the bush having a cylindricalportion 54 providing a bore adapted to receive the bearing shaft of therespective gear, and further having a flange portion 56 extendingradially outwardly at an end of the cylindrical portion to provide athrust face 58 adapted to slidingly engage with a side surface of thegear. Each block also has a backing layer 60 covering a radially outersurface of the cylindrical portion and a rear face of the flangeportion, the backing layer being formed of relatively less dense alloy(e.g. aluminium alloy) compared to the antifriction alloy. The bore andthrust face may be e.g. lead-indium plated or coated with dry filmlubricant to improve tribological compatibility.

Each solid bearing block 48 further has a bearing bridge 62 which sealshigh pressure fuel from low pressure fuel. The bearing bridge can be forexample an aluminium bronze insert. In addition to providing sealingbetween high pressure fuel and low pressure fuel, the insert can help toresist cavitation erosion of the bearing blocks, as described in U.S.Pat. No. 6,716,010 and U.S. Pat. No. 7,607,906. Portions of the thrustface at opposite sides of the bearing bridge are set back so as toprovide side ports 64 which allow fuel into and out of the meshinggears.

Each solid bearing block 48 also has a recess in the surface of the borewhich forms, in use, a hydrostatic pad 66 for the supply of fluid to theinterface between the bore surface and the bearing shaft. A dowel 68between the bearing blocks engages with corresponding recesses in thesides of the block to limit relative movement of the blocks.

FIG. 6 shows a perspective view of two of the pressure loaded bearingblocks 50 which form a pressure loaded bearing set. Similarly to thesolid bearing blocks, each pressure loaded bearing block is formed of anantifriction alloy bush 52, covered with a backing layer 60. Like thesolid bearing blocks, although not entirely visible in FIG. 6, eachpressure loaded bearing block has a cylindrical portion, flange portion,bearing bridge, side ports and hydrostatic pad. Further, the twopressure loaded bearing blocks are fixed together with a dowel andrecess arrangement.

Springs 70 are set into respective recesses 72 on the rear of eachpressure loaded bearing block 50. These springs provide mechanical loadwhich can increase the force of engagement between the bearing thrustfaces and respective side surfaces of the gears. ‘O’ ring seals 74 areprovided at the rear of the bearing blocks to seal high pressure fromlow pressure. Each seal is carried by an offset nose 73 of the block,with the spring recesses at one side of the block being formed in thenose and the spring recesses at the other side of the block being formedoutside the nose. This arrangement determines the hydraulic load andmoment forcing the bearings against the sides of the gears.

Under extreme loads the bores in the bearing blocks can experiencereductions in diameter, the greatest reductions occurring close to thethrust faces of the bearing blocks, where the complete diameter of eachbearing block is formed of lead bronze due to a flange portion of thebush. Such excessive deformation results in the clearance between thegear journal and the bearing bore being reduced, which cancorrespondingly lead to overheating and eventually damage of the gearjournal and/or the bearing block.

FIGS. 7A and 7B show a longitudinal cross section and a transverse crosssection on plane A-A through a solid bearing block 48 which has featuresto reduce or eliminate such deformation of the bearing bore. Similarfeatures can also be incorporated in a pressure-loaded bearing block 50.In particular, the block 48 has an annular gallery 75 embedded in theflange portion 56 of the bush 52 such that the gallery is spaced fromthe surface of the bore and from the thrust face. The annular gallery isspaced a distance of between 2% and 20% or less of the axial length ofthe bush from the thrust face, as measured in the axial direction of thebore, and is also spaced a distance radially outwards of the surface ofthe bore, which distance is at least 15% of the radial distance betweenthat surface and the outer radius of the flange portion. The gallerysurrounds, and is axisymmetric about, the axis of the bore. The galleryhas an inlet 78 (not shown in FIG. 7—but see FIG. 8 discussed below) andan outlet 80 such that in use, fluid flows from the inlet, through thegallery to provide cooling of the thrust face 58, and then to theoutlet. More specifically, the cooling fluid which, in use, flowsthrough the annular gallery, is the fuel which is being pumped by thegear pump of which the bearing block is a component. By providingcooling the thrust face of the bearing block in the form of the annulargallery embedded in the flange portion 56 of the bearing block bush 52,heat transfer away from the thrust face 58 of the bearing block may beimproved. In turn, by increasing the heat transfer away from the thrustface, the load carrying capacity of the face may be correspondinglyimproved. This can allow materials with increased strength but lessinherent load capability to be used. For example, 20 wt % lead bronzemay be used as the antifriction alloy of the bush rather than 30 wt %lead bronze. Use of such materials with increased strength can therebyhelp to prevent significant radial deformation of the bore of thebearing block.

The outlet 80 from the annular gallery 75 is formed in a recess in thesurface of the bore. The recess forms, in use, the hydrostatic pad 66which is fed with the pumped fuel via the annular gallery. Thehydrostatic pressure of the fuel at the pad can assist in achievingfull-film lubrication at the journal element of the bearing block (i.e.inside the bearing bore, between the gear bearing shaft and the boresurface). The flow exhausting from the hydrostatic pad may also provideforced cooling of the journal element. By using the flow in the annulargallery to lubricate the journal element and provide such forced coolingafter cooling the thrust face, the total amount of flow for cooling andlubrication that needs to be diverted from the discharge of the gearpump can be reduced.

The annular gallery can be located at the same radial position as thegear teeth root circle diameter D of the corresponding gear. The coolingof the thrust face 58 thus occurs directly behind the gear teeth rootcircle diameter D, within the gear teeth root circle footprint. Thisregion of the thrust face has limited fuel flow across it duringoperation. Location of the annular gallery at the same radial positionas the gear teeth root circle can help to ensure cooling of thistypically problematic region of the thrust face.

FIG. 8 shows a plan view of a portion of the thrust face of the bearingblock at the bearing bridge insert 62, which intersects the annulargallery 75. A passage 82 formed in the insert allows fluid communicationof two legs of the annular gallery through the insert. This can help toensure that both legs of the annular gallery are fed with anapproximately equal delivery pressure and are not left stagnant. Thebearing bridge insert 62 provides partitioning between a high pressureHP and a low pressure LP side of the gear and also provides locallyincreased cavitation erosion resistance. The bearing bridge insert istypically formed of aluminium bronze. The inlet 78 of the annulargallery can be formed in a side port portion of the thrust face, on thehigh pressure side of the bearing bridge block. In this way, the pumpedfuel flows from the inlet into the annular gallery at a relatively highpressure.

The gallery 75 is formed by partially filling an annular groove in therear face of the flange portion 56 of the bush 52 with an annular insert76 having a depth less than the depth of the groove, thereby forming anenclosed volume constituting the gallery between a bottom face of theinsert and the floor of the groove. The groove may be formed bytrepanning, or any other suitable processing step. The flow velocitywithin the cooling gallery is governed by the cross-sectional area ofthe gallery and thus by its axial depth. However, the depth of thetrepanned groove or depth of the insert inserted into the groove neednot be constant around the circumference of the gallery, such that thecross-sectional area of the gallery can be changed around the bush toalter the strength of the bush, as well as to adjust the flow velocityand heat transfer coefficient. Machining considerations and the strengthof the antifriction alloy are factors in determining the dimensions andlocation of the gallery. The insert can conveniently be joined to thebush by soldering or other suitable process. The back surface of theinsert can be machined at the same time as the rear face of the flangeportion and outer surface of the cylindrical portion 54 to ensure asmooth and continuous surface to accept the aluminium alloy of thebacking layer 60, which is sprayed onto the bush.

The material from which the annular insert 76 is formed may convenientlybe formed of the same antifriction alloy as that used to make the bush52.

Another option, however, is for the insert 76 to be a stiffening ringformed of a material having a higher stiffness than the antifrictionalloy. In particular, the stiffening ring may be made of a materialwhich has a higher elastic modulus and generally also a higher yieldstrength than the antifriction alloy of the bush. However, thecoefficient of thermal expansion (CTE) of the stiffening ring materialis preferably closely matched to the CTE of the antifriction alloy.

The location of the stiffening ring 76 is selected to avoid beingexposed or damaged during further processing steps, including, forexample machining of the side ports in the thrust face 58. Inparticular, breaking the ring may reduce its strength, and exposure ofthe ring can produce undesirable leakage paths in the bush 52.

FIG. 9 shows a graph of modelled stress normalised by yield stressagainst normalised radial distance r/r₀ for a bearing block having suchan annular stiffening ring, r₀ being the outer radius of the flangeportion 56. The model uses an axisymmetric solution of thick ringtheory. It ignores stresses in the axial direction, as well as thermalstrains. It also assumes that no stresses are induced in the blockduring assembly. This implies that the stiffening ring is retained inthe recess by a fixing process, such as soldering, that induces fewinternal stresses. Advantageously, soldering can also help to form afuel tight seal between the ring and the bush. However, other fixingprocesses, such as interference fitting, are not excluded.

The modelling assumes a high, but not atypical, uniform externalpressure typical of running the corresponding gear pump on a pressurerelief valve. The internal pressure distribution resulting fromhydrodynamic, elastohydrodynamic and hydrostatic lubrication may alsocontribute to an “ovalising” deformation, but this is ignored by themodelling. Yielding is modelled using the Tresca criterion, whichassumes that yielding occurs when the difference between the radial andhoop stresses exceeds the yield stress as measured in a uniaxialcompression test.

In FIG. 9, a steel stiffening ring having a thickness which isapproximately 25% of the radial distance from the surface of the bearingbore to r₀ is modelled. The inside radius of the ring is at a radialspacing from the surface of the bearing bore, which spacing is alsoapproximately 25% of the same radial distance. This is to allow for asmall amount of elastic and plastic deformation in the region of thebush close to the bearing bore in order to accommodate bending andmisalignment of the bearing shaft, and allow the bush and shaft tobecome conformal with each other.

The normalised stress values highlighted by the block arrows in FIG. 9,show that by introducing the steel stiffening ring, the modellingpredicts a reduction in the normalised Tresca stress (i.e. (radialstress minus hoop stress)/yield stress) at the bore surface from 2.636to 1.465. This suggests that the stiffening ring should significantlyreduce the amount of deformation of the bearing bore.

In FIG. 7 and FIG. 9, the ring 76 is axisymmetric about the centre ofthe bearing shaft. However, the stiffening effect can be tuned by havingsome parts of the ring closer to the thrust face 58 than other parts, orby having the ring thicker in some parts than in others.

The ring 76 should be close to the bore surface where maximumdeformation occurs, but not so close that it prevents the bush fromaccommodating bending and misalignment of the gear shaft. For example,approximate values for a typical gear pump bearing block may be asfollows: a typical block 48 may be about 70 mm in diameter and about 40mm long, and the flange portion 56 may have an axial thickness of 8-10mm at its outer edge, increasing to an axial thickness of 20-25 mm whereit meets the outer surface of the cylindrical portion 54. In such anexample, the ring may be 3-6 mm in radial thickness and may be spacedabout 3 mm from the thrust face and also about 3 mm from the boresurface. However, these values are in no way limiting, and the actualdimensions of a gear pump bearing block may be selected as appropriateto optimise properties of the bearing block when in use.

FIGS. 10A and 10B show a longitudinal cross section and a transversecross section on plane B-B through a variant solid bearing block 48which also has features to reduce or eliminate deformation of thebearing bore. Again, similar features can be incorporated in a variantpressure-loaded bearing block 50. Corresponding or identical features inFIGS. 7 and 10 have the same reference numbers.

In FIG. 10, the gallery 75 is formed by partially filling the annulargroove in the rear face of the flange portion 56 of the bush 52 with anannular insert 76, which may conveniently be formed of the sameantifriction alloy as that used to make the bush 52. However, the block48 also has a plurality of circumferentially spaced stiffening rods orpegs 176. The stiffening rods are arranged in an annular formation witha gap at the bottom sector, as shown in FIG. 10B, to accommodate sideports formed in the thrust face 58. Moreover, the rods are shown in FIG.10B having different sizes and irregular spacings, but in other blocksthe rod sizes may be more or less equal, and the spacings may be more orless regular. In general parameters such as the position, regularity ofspacing, depth, number and size of rods may be determined by FiniteElement (FE) stress analysis. By varying such parameters, the radialelastic modulus of the bearing block can be varied to give improvedperformance.

The stiffening rods 176 surround the axis of the bore, and arepositioned at approximately equal radial distances from the bore axis,although this is not essential. For example, varying the radialdistances from the bore axis may allow for variation in the radialelastic modulus of the bearing block. The rods are also spaced from boththe surface of the bore and from the thrust face 58 of the bearingblock. Like the annular stiffening ring, the stiffening rods are made ofa material (e.g. steel) which has a higher elastic modulus and generallyalso a higher yield strength than the (e.g. lead-bronze) antifrictionalloy of the bush. The CTE of the stiffening rod material is preferablyclosely matched to the CTE of the antifriction alloy.

The rods 176 are located in respective recesses formed in the rear faceof the flange portion 56 of the bush, and extend therefrom in the axialdirection of the bore into the backing layer 60 of the block, where theyare affixed with plugs 84, which may be, for example, screwed plugs orLee Plugs™. In this way, the rods may transfer some of the load from thelead-bronze flange portion, which tends to suffer from greatest radialdeformation, into the generally stronger aluminium alloy backing layer60 e.g. by a combination of shear and bending. Hot isostatic pressingcan be used to improve the contact between the rods and the lead-bronzeand aluminium alloy, and hence improve load sharing and reduce fluidleakage paths.

Another option relevant to both variants discussed above, is to co-castthe stiffening member(s) with the lead-bronze of the bush, e.g.similarly to how an aluminium bronze insert bearing bridge 62 can beco-cast, as described in U.S. Pat. No. 6,716,010.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

What is claimed is:
 1. A gear pump bearing block having: a bush formedof antifriction alloy, the bush having a cylindrical portion providing abore adapted to receive a bearing shaft of a gear of the pump, andfurther having a flange portion extending radially outwardly at an endof the cylindrical portion to provide a thrust face adapted to slidinglyengage with a side surface of the gear; a backing layer covering aradially outer surface of the cylindrical portion and a rear face of theflange portion, the backing layer being formed of relatively less densealloy compared to the antifriction alloy; an annular gallery embedded inthe flange portion such that the gallery is spaced from the surface ofthe bore and from the thrust face, the gallery surrounding the bore; andan inlet to and an outlet from the annular gallery such that, in use,fluid flows from the inlet, through the annular gallery to providecooling of the thrust face, and then to the outlet.
 2. A gear pumpbearing block according to claim 1, wherein the gear pump bearing blockfurther has a recess in the surface of the bore which forms, in use, ahydrostatic pad for the supply of fluid to the interface between thebore surface and the bearing shaft, the outlet from the annular gallerybeing formed in the recess
 3. A gear pump bearing block according toclaim 1, wherein the inlet to the annular gallery is formed at thethrust face.
 4. A gear pump bearing block according to claim 1, furtherhaving a bearing bridge insert at the thrust face partitioning highpressure and low pressure sides of the gear and providing locallyincreased cavitation erosion resistance, the inlet to the annulargallery being formed on the high pressure side of the bearing bridgeinsert.
 5. A gear pump bearing block according to claim 1, wherein theannular gallery is located at the same radial position as a gear teethroot circle of the gear which, in use, slidingly engages with the thrustface.
 6. A gear pump bearing block according to claim 1, wherein theannular gallery is located 2% or more and/or 20% or less of the axiallength of the bush from the thrust face, as measured in the axialdirection of the bore.
 7. A gear pump bearing block according to claim1, wherein the annular gallery is spaced a distance radially outwards ofthe surface of the bore, which distance is at least 15% of the radialdistance between that surface and the outer radius of the flangeportion.
 8. A gear pump bearing block according to claim 1, wherein theannular gallery is formed by partially filling an annular groove formedin the rear face of the flange portion with an annular insert having adepth less than the depth of the groove, thereby forming an enclosedvolume constituting the annular gallery between a bottom face of theinsert and the floor of the groove.
 9. A gear pump bearing blockaccording to claim 8, wherein the insert is formed of the antifrictionalloy.
 10. A gear pump bearing block according to claim 1 further havingan annular formation of one or more stiffening members, the formationsurrounding the bore, the, or each, stiffening member being embedded inthe flange portion, and the, or each, stiffening member being formed ofa material having a higher elastic modulus than the antifriction alloy.11. A method of manufacturing a gear pump bearing block, the methodincluding steps of: providing a bush formed of antifriction alloy, thebush having a cylindrical portion providing a bore adapted to receive abearing shaft of a gear of the pump, and further having a flange portionextending radially outwardly at an end of the cylindrical portion toprovide a thrust face adapted to slidingly engage with a side surface ofthe gear; forming an annular groove in the rear face of the flangeportion and surrounding the bore, the groove being spaced from thesurface of the bore and from the thrust face; partially filling theannular groove with an annular insert having a depth less than the depthof the groove, thereby forming an enclosed volume constituting anannular gallery between a bottom face of the insert and the floor of thegroove; forming an inlet to and an outlet from the annular gallery suchthat, in use, fluid flows from the inlet, through the annular gallery toprovide cooling of the thrust face, and then to the outlet; and coveringa radially outer surface of the cylindrical portion and the rear face ofthe flange portion of the bush with a backing layer, the backing layerbeing formed of relatively less dense alloy compared to the antifrictionalloy.
 12. A method of manufacturing a gear pump bearing block accordingto claim 12, wherein the step of covering the radially outer surface ofthe cylindrical portion and the rear face of the flange portion of thebush with the backing layer is performed by thermally-spraying therelatively less dense alloy onto the bush.
 13. A gear pump having one ormore gears with bearing shafts supported by respective gear pump bearingblocks of claim
 1. 14. A fuel supply system of a gas turbine enginehaving the gear pump according to claim 13 for pumping fuel.
 15. A gasturbine engine having the fuel supply system of claim 14.